And so if you have a particular nucleotide at one position, say an A, and when it mutates it will change to a G, a C, or a T, just something else. And now, if you look over a long enough evolutionary period of time, that particular point in the DNA segment has a probability of mutating again. And so, and because there's only three choices that it can mutate to, and be seen as a difference, there's a high chance that it will mutate right back to what it was before. So you could have an A mutate to a G, then the G mutates back to an A. And so you've had two mutations and yet, if you only compare, if all you see is an A in one species, and an A in the other, you will say there've been no mutations. So what we actually observe, the number of differences that we count is an underestimate, of the number of mutations that have actually occurred.